1. Field of the Invention
Examples embodiments of the present invention relate to a method for photoculturing and harvesting microalgae, in which a semi-continuous culture method that uses a reactor and a flocculation/recovery tank separate therefrom is applied, and microalgae are cultured using a light emitting diode (LED) as a light source and harvested using a natural polymer flocculant and air microbubbles.
2. Description of Related Art
Generally, the term “microalgae” refers collectively to unicellular organisms that have photosynthetic pigments and are photosynthesized. Microalgae can grow in the presence of a suitable amount of light and dissolved nutrients and can be utilized in various applications, including the production of biomass and biofuel, the improvement of atmospheric and aquatic environments, etc.
Specifically, with respect to the production of biomass by culture of microalgae and the industrial application thereof, microalgae have a characteristic in that they use sunlight as an energy source to capture carbon dioxide and produce useful substances. Studies on the production of industrially important useful substances by culture of such microalgae have been actively conducted. Generally, the term “biomass” refers to organic materials that are produced by photosynthesis. Microalgae are cultured using photosynthesis, and the biomass and industrially useful substances produced by microalgae are used in various applications, including health supplement foods, feed, cosmetic products, raw materials for development of new drugs, transportation fuels and energy, etc.
Studies on the production of biofuels using microalgae have been conducted in various ways according to environmentally friendly policies worldwide. The term “biodiesel” refers to more than 95% pure fatty acid methylesters made by reaction of animal/vegetable oils with alcohols. Biodiesel has physical/chemical properties almost similar to those of diesel oil, and thus can substitute for diesel oil. Meanwhile, high-calorie lipids are required to produce diesel from microalgae. In connection with this, Chlorella has high biomass productivity and a high lipid content of 28-32%, even though the lipid content thereof is lower than those of Nannochloropsis sp. (31-68%) and Botryococcus sp. (29-75%). However, due to strict culture conditions, Chlorella is difficult to culture. In addition, Botryococcus sp. has the highest lipid content among microalgae known to date, but has low cell growth rate and low biomass productivity. For this reason, Chlorella is a strain suitable for culturing microalgae using a high-efficiency reactor, because it has advantages in that it has high growth rate and is easy to culture.
With respect to the improvement of atmospheric and aquatic environments by culture of microalgae, the worldwide average temperature rose by 0.74° C. over the past 100 years (1906 to 2005) due to carbon dioxide, the mean sea level rose by 1.8 mm annually over a period ranging from the year 1961 to the year 1993, and the atmospheric CO2 concentration increased by 30% (from 290 ppm to 360 ppm over the past one century. It is expected that, in the 21st century, the atmospheric CO2 concentration will exceed 550 ppm and the average surface temperature of the earth will increase by 1.9-4.4° C. Thus, with respect to the reduction in CO2 by carbon dioxide capture mechanism and Chlorella culture, microalgae have substantially the same photosynthesis mechanism as higher plants. However, unlike a plant group that uses atmospheric inorganic carbon as a major carbon source, aquatic microalgae can obtain CO2 from dissolved inorganic carbon (DIC). For photosynthesis, microalgae have the photosynthetic enzyme RUBISCO (ribulose bisphosphate carboxylase/oxygenase), and the diffusion of inorganic carbon to this enzyme acts as an important factor that determines the photosynthesis ability. RUBISCO has both carboxylase activity and oxygenase activity, and the two activities competitively appear depending on the concentrations of CO2 and O2. The CO2 affinity of RUBISCO is originally low, but microalgae can show a high photosynthesis rate by a mechanism that increases the intracellular CO2 concentration. Migration of CO2 through the various membranes of microalgal cells in this CO2 concentrating mechanism (CCM) is associated with various carbonic anhydrase enzymes that catalyze the conversion of CO2 to HCO3− (CO2+H2O→HCO3−+H+).
Meanwhile, many studies on the production of biomass and the removal of nutrient salts from wastewater by culture of microalgae have been conducted in order to achieve two goals: free supply of the phosphorus and nitrogen required for culture of microalgae, and water purification. Thus, with respect to improving water quality by culture of microalgae, sewage in Korea has a nitrogen content lower than a carbon content, and for this reason, the efficiency with which nitrogen is removed from sewage by a conventional sewage/wastewater treatment method using microorganisms is low. In other words, if sewage is discharged to the aquatic system in a state in which nitrogen was not sufficiently removed therefrom, economic damage and ecosystem disruption can be caused by eutrophication due to nitrogen. Microaigae are microorganisms that grow by photosynthesis using carbon dioxide as a carbon source without needing to supply an organic carbon source, and thus can capture nitrogen from wastewater having a low C/N ratio. In other words, it was reported that, when algal treatment that is a method of treating nutrients contained in livestock wastewater using microalgae is linked with existing treatment facilities, there will be various synergistic effects. Also, there was a report on the development of a medium capable of enhancing the productivities of Chlorella ovalis and Dunaliella parva using treated water fermented by humus microorganisms for the purpose of recycling livestock manure that deteriorates water quality. Moreover, there was a report on the research and development of an algae culture process using Chlorella sp. HA-1 to remove nutrient salts such as nitrogen and phosphorus, which are discharged from livestock wastewater to cause the eutrophication of rivers. In addition, studies were conducted to examine the ability of Chlorella Kessleri to remove nitrogen from wastewater having a low C/N ratio and to examine whether an apparatus can remove nitrogen from wastewater using a single strain of Chlorella. 
Meanwhile, general methods for culturing microalgae include a method employing an open culture system. In this conventional microalgae culturing method, as shown in FIG. 1, microalgae are cultured in a natural environment such as an open pond rich in nutrient sources. Such a conventional open type microalgae culturing method has advantages in that initial capital investment and operating costs are low and the system is easy to maintain and repair. However, the conventional open type microalgae culturing method entails shortcomings in that the growth of microalgae is slow because natural light that is not effectively transferred into the culture system, the growth yield of microalgae is low, the biomass of microalgae is unstable due to contamination, nutrient sources for microalgae are non-uniformly distributed, and a large installation space is required to remove a large amount of carbon dioxide. In addition, the precipitation of microalgae occurs, and in the case of a circular pond, effective agitation of the central portion is difficult, and for this reason, the effective growth of microalgae is significantly reduced, resulting in a decrease in the productivity of microalgae. At present, the open culture systems are constructed in a raceway shape, and the raceway-shaped pond comprises one channel or several channels communicating with each other, has a depth of about 0.3 m, is made of a material such as concrete or plastic, and is provided with a paddle wheel to circulate a culture medium and prevent the precipitation of microalgae. Cooling of the culture medium in the raceway pond is controlled by evaporation, and carbon dioxide that is supplied to increase the production of microalgal biomass is mostly lost into the atmosphere, and thus the raceway pond has low photosynthesis efficiency compared to a photobioreactor. In addition, the productivity of microalgae is greatly affected by contamination with other microalgae and microorganisms.
Meanwhile, a method employing a closed culture system is shown in FIG. 2. As shown therein, the closed culture system comprises a tubular photobioreactor or a flat-plate photobioreactor and has advantages in that effective sterilization is possible, the transfer of gas is easy, and the system has a simple structure, and thus can be easily installed in any place. The tubular photobioreactor is mainly made of a glass or plastic material and has various advantages, including high agitation ability, effective sterilization, easiness of gas transfer, and easiness of spatial installation. However, it has a structural disadvantage in that, as the diameter of the tube is increased for mass culture, the surface lighting area per volume is reduced. Due to this disadvantage, an effective agitation system or an artificial lighting system is required to be additionally provided, and in this case, the length of the tube is limited. A photobioreactor for high-concentration culture is required to have a high ratio of surface area to volume, and the easiest and simplest method that satisfies this requirement is a flat-plate photobioreactor. Due to this advantage, studies on the flat-plate photobioreactor have been actively conducted. In addition, in the case of an optical fiber reactor, the efficiency with which light energy is increased by irradiating light into the reactor through optical fiber, and it is possible to obtain high carbon dioxide capture efficiency compared to that in other reactors. However, there is a disadvantage in that initial capital investment is excessively high due to expensive optical fiber and facilities.
Meanwhile, in culture methods that use different growth conditions, a photoautotrophic method requires only water, light and fundamental nutrients for the maximum growth, an autotrophic method uses carbon dioxide as a carbon source without irradiating light, a heterotrophic method performs culture in a tank in an aseptic environment under a dark condition using glucose as a carbon source without irradiating light, and a mixotrophic method uses an organic carbonic acid such as acetic acid while radiating light.
In view of the foregoing, the closed culture system that is less influenced by climate is effective for use in Korea, because the differences in temperature and rainfall between days and seasons in Korea are severe. Particularly, the open culture system is not suitable for use in Korea, because it requires a large area of the site.
In the prior art related to the present invention, patent document 1 discloses a continuous photoreactor for mass production of microalgae, which uses a fluorescent lamp as a light source.
When a fluorescent lamp is used as a light source, the photoconversion efficiency is higher than that of a glow lamp (8%), but is lower than that of LED (25-30%), and for this reason, when the fluorescent lamp emits light, the surface temperature thereof approaches 110° C., and thus it cannot be placed near growing plants or a microalgae incubator. In addition, there is a shortcoming in that the light efficiency is rapidly reduced because the intensity of light is generally inversely proportional to the distance from the light source.
In addition, there are problems in that a separate cooling system is required to be provided in order to solve the above problems and in that a separate filter for removing a harmful wavelength should be provided because the fluorescent lamp does not emit light having a wavelength unnecessary for culture.
Meanwhile, most microalgae are not easy to separate from a culture medium, because they are present at low concentration in the medium and have a size of 30 μm or less and the density thereof is slightly higher than that of water. Thus, one of important problems to be solved in a process of producing useful substances by mass production of microalgae is economic harvesting. A suitable harvesting method varies depending on the kind of algae and the intended use of the useful substances to be obtained from algae and is generally performed using complex processes such as filtration, sedimentation, flotation, centrifugation, flocculation and the like.
In the prior art related to the present invention, patent document 2 is directed to a photobioreactor for high-density culture of microalgae and a method for culturing and harvesting microalgae using the same and discloses adding flocculants (CaCl2 and FeCl3) to a culture medium to induce flocculation after completion of the culture of microalgae, followed by harvesting of the microalgae.
The flocculation method is a method in which microalgae are harvested using chemical flocculants or bioflocculation to separate microalgae in a culture medium from the aqueous solution. The surface of microalgae is positively charged, and thus the microalgal particles are present in a state in which they are suspended in the aqueous solution. The chemical flocculants serve to neutralize the surface charge of such microalgae to reduce the repulsive power between the particles to thereby promote the bonding between the particles. However, if the chemical flocculants are used, chemicals are included in microalgae to limit the use of the harvested microalgae. In addition, the biological flocculation method has shortcomings in that it is time-consuming and requires a somewhat complex process, compared to the flocculation method that uses the chemical flocculants.
Further, patent document 3 is directed to a photobioreactor for culture of microalgae and discloses harvesting cultured microalgae using a centrifuge in order to increase the absorption of carbon dioxide.
It was reported that, when the centrifugation method is used, the harvesting cost accounts for about 20-30% of the total cost of the microalgae process, even though it depends on the kind of microalgae, cell concentration and a culture method. The harvesting cost is generally calculated by averaging the costs incurred in the process of separating microalgae from the culture medium using continuous centrifugation as opposed to batchwise centrifugation. In other words, continuous centrifugation consumes a large amount of energy, because the concentration of microalgae in the culture medium is low. It was reported that energy required to separate microalgae from a culture medium containing 0.04-4% (on a dry weight basis) microalgae by centrifugation reaches 1.3 kWh/m3 and that energy of about 8 kWh/m3 is consumed to make a dry weight of 22% by centrifugation.
When microalgae are harvested using a membrane filter, power required to obtain a harvesting efficiency of 70% is no more than 0.25 kWh/m3, and thus the use of centrifugation in the production of biodiesel has low cost effectiveness.
Recently, technology of floating the microalgae of the culture medium by applying a vacuum was also developed. It was reported that, when the flotation method that uses a vacuum is used, energy required to harvest microalgae is 0.2 kWh/kg DW, which is 10-100 times lower than those of the existing flocculation method and centrifugation method. However, the flotation method has a problem in that the initial capital investment is higher than those of other methods.
In addition, in patent documents 1 to 1 3, the culture, flocculation and recovery of microalgae are performed in a single reactor, and thus microalgae are harvested in large amounts. However, the above patent documents encounter problems in that the culture and harvesting of microalgae are time-consuming and the concentration of microalgae is low.